The networking of control units, sensor systems and actuator systems with the aid of a communication system or data transmission system and a communication link, e.g., in the form of a bus system, has increased dramatically in recent years in modern motor vehicles, but also in other sectors, for example, in machine construction, especially in the field of machine tools, and in automation. In this context, synergistic effects may be achieved by the distribution of functions to a plurality of users, e.g., control units, of the communication system. These are called distributed systems.
Increasingly, the communication between various users of such a data transmission system is taking place via a bus system. Communication traffic on the bus system, access and receive mechanisms, as well as error handling are regulated by a protocol. For instance, one known protocol is the FlexRay protocol, presently based on the FlexRay protocol specification v2.1. FlexRay is a rapid, deterministic and fault-tolerant bus system, particularly for use in motor vehicles. The FlexRay protocol operates according to the principle of Time Division Multiple Access (TDMA), in which the users or the messages to be transmitted are assigned fixed time slots in which they have exclusive access to the communication link. The time slots repeat in a fixed cycle, so that the instant at which a message is transmitted via the bus can be predicted exactly, and the bus access takes place deterministically.
To optimally utilize the bandwidth for the transmission of messages on the bus system, FlexRay subdivides the cycle into a static and a dynamic part, that is, into a static and a dynamic segment. The fixed time slots are in the static part at the beginning of a bus cycle. In the dynamic part, the time slots are preset dynamically. Therein, the exclusive bus access is now in each case only permitted for a brief time, for the duration of at least one so-called minislot. Only if a bus access takes place within a minislot is the time slot lengthened by the time necessary for the access. Consequently, bandwidth is thus only used up if it is also actually needed. FlexRay communicates via one or two physically separate lines with a data rate in each instance of 10 Mbit/sec. maximum. However, FlexRay can, of course, be operated with lower data rates as well. The two channels correspond to the physical layer, in particular of the so-called OSI (open system architecture) layer model. They are used chiefly for the redundant and therefore fault-tolerant transmission of messages, but can also transmit different messages, which means the data rate could then double. It is also conceivable that the signal transmitted via the connecting lines results from the difference of signals transmitted via the two lines. The physical layer is developed in such a way that it permits an electrical, but also optical transmission of the signal or signals via the line(s) or a transmission in another way.
To realize synchronous functions and to optimize the bandwidth by small intervals between two messages, the users in the communication network need a common time base, the so-called global time. To synchronize local clocks of the users, synchronization messages are transmitted in the static part of the cycle, the local clock time of a user being corrected with the aid of a special algorithm corresponding to the FlexRay specification, so that all local clocks run in synchronization with a global clock.
In transmitting data or messages via such a bus system, pulses become distorted because high-to-low or low-to-high edges on the transmission path are delayed by different amounts. If the transmitted pulse is sampled a multiple number of times (for example, n times per bit) in the receiver with the sample clock (the so-called sampling rate) existing there, then the position of the sampling point, i.e., the selection of exactly one of these n sampling values, decides whether the datum is sampled correctly or incorrectly. This is particularly difficult when the sampling instant refers to one edge of the signal, and relative thereto, a plurality of binary data values (bits) from the transmitter are also evaluated over many periods of the sample clock. In addition to a pulse distortion, the clock-frequency deviation between transmitter and receiver has an effect here as well. In this context, the signal to be sampled may be preconditioned in order, for example, to filter out short-duration interferences. Such a filter may then be implemented by evaluating a plurality of sampled signals in the time sequence with a majority decision (so-called voting). Particularly in the case of the FlexRay protocol specification in which, given n network nodes, there can be
  2  ⁢            ∑              i        =        0                    n        -        1              ⁢    i  different transmission paths (each conceivable path has 2 transmitter-receiver combinations), it has turned out that the fixed determination of the sampling instant without taking the asymmetrical delays on the different transmission paths into consideration leads to problems in the timing. The delay between the rising and falling edge of a signal is also known as pulse distortion or asymmetrical delay.
Asymmetrical delays may have both systematic and stochastic causes. In the FlexRay protocol, systematic delays affect only the rising edges, since synchronization is carried out to the falling edges. Stochastic delays have effects both on the rising and on the falling edges, and are caused by noise occurrences or EMC jitter.
Owing to the fixed selection of the sampling instant per bit (for example, given n sampling values per bit at n/2, in the middle of a bit), both the influence of the asymmetrical distortion as well as the frequency deviation and the additional time discretization due to the sampling are a problem, and place high demands on the transmission channel. Increasing the edge steepness in order to reduce the asymmetrical delays would indeed bring advantages for the timing, but would on the other hand require technically more demanding and thus costlier components, and in addition would unfavorably influence the EMC performance of the data transmission system. Therefore, it is sometimes more advantageous not to select the edge steepness to be so great; however, depending on the pulse distortion, one runs the risk of evaluating the wrong datum either at the one or the other bit boundary.
In addition, in the implementation of FlexRay data transmission systems, particularly in the case of complex systems including a plurality of star couplers and passive networks, it has turned out that the asymmetrical delay times occurring there are so great that they exceed a time budget predefined by the FlexRay protocol. According to the FlexRay protocol, a sample counter is synchronized with the falling BSS (byte start sequence) edge, that is, is set back to 1. Sampling is carried out at a counter reading of 5. In the case of an 8-fold oversampling as presently provided in FlexRay, between the sampling instant (5th sampling value) and the 8th sampling value, 3 sample clocks thus still remain which, given a communication controller clock pulse of 80 MHz, in each case correspond to 12.5 ns, thus in total to a time budget of 37.5 ns. This time budget is actually used for the compensation of asymmetrical delays because of the difference of falling to rising edge steepness. If, however, —as can be the case in complex network topologies—the asymmetrical delay exceeds the time budget provided, the result is that, given a sampling at the 5th sample clock (reading of the sample counter at 5), a false value is ascertained because the bit which should actually have been sampled was already present at an earlier instant due to the asymmetrical delay, and is no longer present owing to the early edge change. An analogous treatment holds true for an asymmetrical delay retarded in time. A time budget of 4 sample clocks corresponding to 50 ns is then available. If the time budget is exceeded in a manner advanced or retarded in time, decoding errors result, thus false data are received.
It may be that these decoding errors can be detected by suitable error-detection algorithms, so that it is possible to cause the bit or the entire data frame to be transmitted again. For example, a so-called parity bit or a so-called cyclic redundancy check (CRC) may be used as an error-detection algorithm. However, the disadvantage of a frequent response of the error-detection algorithm lies in the decreased availability of the data transmission system associated with it, since, for example, the incorrect data are transmitted again or are discarded.
In summary, it can be said that demands are made by the FlexRay protocol which the physical layer cannot sustain—at least in the case of complex network topologies.